Method for making color television picture tubes



Dem 1965 J. GIUFFRIDA 3,222,172

METHOD FOR MAKING COLOR TELEVISION PICTURE TUBES Filed April 27 1955 I 3 Sheets-Sheet 1 INVENTOR. Joseph Giuffrida ATTORNEY 1965 J. GIUFFRIDA 3,222,172

METHOD FOR MAKING COLOR TELEVISION PICTURE TUBES Filed April 27 1955 3 Sheets-Sheet 2 DEFLECTION YOKE 1L) \8 PURITY COIL PLANE I I l4 20 A l I I FIG. 3

INVENTOR. Joseph Giuffrido ATTORNEY Dec. 7, 1965 J. GIUFFRIDA 3,222,172

METHOD FOR MAKING COLOR TELEVISION PICTURE TUBES Filed April 27 1955 3 Sheets-Sheet 5 FIG. 4

INVENTOR. Joseph Giuffrido ATTORNEY United States Patent 3,222,172 METHOD FOR MAKHNG COLOR TELEVISION PICTURE TUBES Joseph Giuffrida, Peabody, Mass, assignor to Columbia Broadcasting System, Inc, a corporation of New York, doing business as CBS-Hytron, a division of Columbia Broadcasting System, Inc., Danvers, Mass.

Filed Apr. 27, 1955, Ser. No. 504,287 8 Claims. (Cl. 96-35) This invention relates to color television in general and to color television picture tubes in particular.

The most practical color television picture tube presently manufactured is the so-called shadow mask tube. The shadow mask tube is a modified cathode ray tube having a multi-colored luminescent screen, a perforated shadow mask and three electron guns. Auxiliary components, not ordinarily considered to be part of the tube itself, are usually mounted on the outside of the tube to control and deflect the electron beams emanating from the electron guns.

The screen is composed of three different luminescent materials or phosphors, each producing one of the additive primary colors. In the tubes presently used, the first phosphor produces the color red when excited by electrons; the second, blue; and the third, green. There are many phosphors known to the art which have the desired color response when bombarded with electrons. Each of the phosphors is separately deposited on discrete portions of the viewing end of the tube bulb to form a symmetrical mosaic. Although the shape of each phosphor element in the mosaic may be varied, it is preferred that each element be circular. The individual elements or dots in the mosaic are arranged in closely spaced triads, each dot in each triad producing one of the primary colors upon excitation.

The shadow mask is usually a thin metallic sheet disposed between the luminescent screen and the electron guns. Apertures are formed in the shadow mask in such a manner that the elements or dots on the screen which produce red may be energized only by electrons which arrive at the screen from a first direction, those which produce green may be energized only by electrons arriving from a second'direction and those which produce blue by electrons arriving from a third direction. In a practical tube, there is one aperture in the shadow mask for each triad of dots on the viewing screen.

The electron guns may be of well-known design, but it is preferred that they be mechanically modified for mounting adjacent each other. The electron beams from the guns will then be substantially parallel to each other and equally spaced around the longitudinal axis of the tube. External means are usually provided to cause the electron beams to converge simultaneously on the same aperture in the shadow mask. Thus, each electron beam approaches the shadow mask from a different direction. The proper proportioning of the electron guns and the relative size and position of each aperture in the shadow mask with respect to each of the phosphor dots so restricts each electron beam that it energizes only one type of phosphor dot.

Although the theory of operation of a shadow mask tube is relatively simple, experience has proven that a practical tube is very difiicult to manufacture. While each component contributes to the difiiculty of manufacture, perhaps the most difiicult component to make is the multi-color viewing screen. Some idea of the difiiculties involved in making the viewing screen may be obtained if one considers that there are approximately 4,000 phosphor dots per square inch of viewing screen and that the tolerance in position of each dot is approximately one one-thousandth of an inch.

3,222,172 Patented Dec. 7, 1965 lice Several processes, such as silk screening or printing, have been developed to produce luminescent screens for color television picture tubes. However, the presently preferred method of producing a luminescent screen is shown in detail in the co-pending application of Perry and Rowe, Serial No. 387,912, entitled, Process for Forming Color Screens, and assigned to the same assignee as the present application. The cited application discloses a photographic process whereby images of each aperture in the shadow mask are formed on the luminescent screen by light emanating from a point source. Each dot on the screen, an aperture in the shadow mask and the beam deflection center of the corresponding electron beam are in an optically straight line. Such positioning, in turn, allows registration of any given electron beam on only one color-producing array of dots, provided the trajectory of each electron beam is a straight line.

Unfortunately, the trajectory of each electron beam in a finished tube is not a straight line. The magnetic field of the earth exerts a force on each electron beam of such magnitude and direction that each beam follows a curved path. In all practical tubes used in the northern hemisphere, the path is essentially circular, bending to the right as observed from the electron guns looking toward the screen. The approximate circularity of the path is caused by the vertical component of the earths magnetic field.

Various ways have been tried to minimize the effect of the earths magnetic field. The most obvious way, of course, is to shield the electron beams from the earths field. However, experience has shown that the best magnetic shields, even those made from very permeable metals such as mu metal, are expensive to make and difiicult to use. In fact, adequate shielding has never been attained in practice.

The practice of two of the leading manufacturers of color picture tubes may be cited to illustrate compensation of the curvature of the electron beams in a color picture tube by planned distortion of the electron beams, rather than by magnetic shielding.

One of the leaders in the field places magnets on the outside of the tube adjacent the periphery of the viewing screen to form a so-called color equalizer. The electron beams are allowed to bend under the influence of the earths field until they come into the field of the peripheral magnets. The peripheral magnets are so arranged and polarized that the path of each electron beam may be sharply bent near the viewing screen to direct each beam to its corresponding phosphor clot. The inherent difliculty of such a system is obvious to one skilled in the art. An absolutely uniform field having a predetermined magnitude and direction must be maintained across an area approximately the size of the viewing screen by adjusting the strength and position of a number of independent magnets. Adjustment of the peripheral magnets to obtain an absolutely uniform field is difficult even under laboratory conditions; after installation in a practical color television receiver, proper adjustment is almost impossible. Moreover, the size of the compensating magnets and their position around the viewing screen forces enlargement of the cabinet in which the tube is installed.

The other manufacturer presently uses a so-called purity coil to obtain color registry. The purity coil is an electromagnet mounted on the neck of the picture tube, so that its field is concentrated along a short distance near the electron guns. The coil is energized with direct current to produce a magnetic field. The intensity and direction of the magnetic field may be adjusted by changing the current and the polarity of the coil current or by rotating the coil around the neck of the tube.

The intensity and direction of the magnetic field, in turn, affect the direction of the electron beams. Although each electron beam is allowed to bend under the influence of the earths field along the greater part of its length, color registration may still be obtained by the proper adjustment of the purity coil field.

The use of a purity coil has some advantages over peripheral magnets, but several different objectionable effects arise. Perhaps the most objectionable effect of a purity coil is that known as neck shadow. Neck shadow causes a serious problem, namely, the loss of a portion of the visible raster. This loss results from interception of the electron beams by the inner wall of the tube at the junction of the neck and the conical part of the tube whenever a certain critical deflection angle is exceeded. Unfortunately, use of a purity coil makes it more likely that the critical deflection angle be exceeded. In fact, in an unshielded, all glass, wide angle tube, neck shadow is almost invariably observed when the purity coil is adjusted to obtain optimum color registration and this phenomenon may even be found in metal cone tubes.

The purity coil is in proximity to and interacts with the deflection, convergence and centering electromagnets usually used with color television picture tubes. The resultant magnetic field is very complex; the design and adjustment of each electromagnet being greatly affected by that of each other electromagnet. Such complexity makes adjustment diflicult for even trained personnel; it is evident that widespread commercial acceptance of color television requires that adjustments be simplified so that untrained persons may perform them.

The shifting of at least one electron beam away from the tube axis caused by the purity coil imposes greater restrictions on the design of the deflection yoke used with the tube. It is almost impossible in practice to obtain an absolutely uniform magnetic field across an area the size of the neck of a color picture tube. Experience has shown that the field intensity of any practical deflection yoke decreases an unpredictable amount as the distance from the geometric center of the deflection yoke increases. Since the purity coil usually shifts one beam farther from the geometric center of the deflection yoke, such a beam will experience less deflection at any given instant than the beams which are closer to the geometric center of the yoke. As a result, the beams do not converge on the shadow mask at all times and the quality of the picture deteriorates. It should be noted that the same difficulty is encountered regardless of the type of deflection utilized in the tube.

Even if the deflection yoke is designed so that an absolutely uniform field is obtained, proper convergence of the electron beams cannot be attained at all times if one beam is moved farther from the tube axis than the others. A moments thought will show that convergence can be attained only when the length of each electron beam is kept equal to the length of each of the other beams at all times. If one of the beams is moved farther from the axis of the tube than the others, such a condition is impossible to meet.

Therefore, it is a general object of this invention to obtain color registration in a color television picture tube without apparatus to compensate for the earths magnetic field.

It is a still further object of this invention to eliminate neck shadow in a color television picture tube.

A still further object of this invention is to improve convergence of the electron beams in a color television picture tube.

Another object of the invention is to provide a process for fabricating a color television picture tube needing no compensation for magnetic effects.

For a better understanding of the invention together with other and further objects, features and advantages, reference should be made to the following description which is to be read in connection with the drawings, in each of which corresponding parts are similarly numbered, and in which:

FIG. 1 is a perspective view, partly cut-away, of a color television picture tube embodying the principles of this invention.

FIG. 2 is a diagram of an electron beam path in a color television picture tube showing in particular how the viewing screen may be offset to compensate for curvature of the electron beams in a color television picture tube.

FIG. 3 is a diagram of a color television picture tube showing how this invention minimizes neck shadow and improves scanning.

FIG. 4 is a perspective view, partly cut-away, of an apparatus by which the principles of this invention may be used to fabricate a tri-color viewing screen.

In general, the present invention consists in a color television picture tube, and a method of producing such a tube, wherein no compensation for the effects of the earths magnetic field is required after the tube is fabricated. In the finished tube, the centers of the phosphor dots do not bear a straight-line relationship to the electron gun with which they are associated. Rather, a line traced from a given gun to any of the phosphor dots which it is designed to energize follows a curved path. The curvature of each line is such that the electron beams from each gun pass through each aperture in the mask at the same angle as the light beam which originally formed the dot, if the photographic process was used.

In the present invention, using the photographic process of producing phosphor dots bearing the desired relationship to the energizing electron guns, the point source of light is offset from the straight-line relationship. The amount of offset is dependent upon the physical size of the tube, the velocity of the electron beams therein, and on the strength of the earths magnetic field. For a better understanding of the structures and processes of the present invention, reference should be made to the following detailed description and to the drawings.

Referring now to FIG. 1, an embodiment of the present invention in a practical tube may be seen. It should be noted that several simplifications, not material to the invention have been made. The electron beam forming, focusing and accelerating elements have not been shown. Standard means to shape and direct the electron beams are well-known in the art and may be applied without change in a tube embodying the concepts of the present invention. Well-known current waveshapes may be applied to the deflection yoke shown in FIG. 1 to obtain horizontal and vertical deflection of the beams. Only one gun and one beam are depicted to prevent undue complication of the figure. The tube shown in FIG. 1 operates in the following manner. An electron gun 11 is positioned in the neck 12 of the tube 13. The electron beam 14 is directed along the length of the neck 12. A magnetic deflection yoke 15 may be mounted on the outside of the neck 12 as shown although the type of deflection means used is not material to the invention. The electron beam 14 is shown for convenience, deflected toward the observer in a horizontal plane. After deflection by the deflection yoke 15, which may be considered, without significant error, to take place at point A, the electron beam 14 passes along the length of the tube 13 through an aperture 16 formed in the shadow mask 17 and impinges on a phosphor dot 18. Both apertures and phosphor dots have been greatly exaggerated in size to show more clearly the working of this invention. The electron circuit is completed through a conducting surface (not shown) on the phosphor dot 18 back to the electron gun 11. The electron beam 14 follows a substantially circular path from the point A through the aperture 16 to the phosphor dot 18. The electron beam 4 also follows an approximately circular trajectory between its point of origin at the electron gun 11 and the point A, the path having the same radius of curvature as the path from the point A to the phosphor dot 18.

The earths magnetic field which imparts the curvature to beam 14 is represented by the arrows 19. The inclination of the arrows 19 to the horizontal represents the dip angle of the earths magnetic field at a latitude of approximately 40 and the direction of the arrows represents the direction of the earths magnetic field 19 in the northern hemisphere. The cross-sectional area of the tube 13 is so small that the field 19 may be assumed to be uniform throughout the tube. A well-known principle of electron motion in a uniform magnetic field may now be applied to determine the trajectory of the electron beam 14. The principle is that if the original velocity of an electron is not normal to the direction of a magnetic field, the velocity component of an electron parallel to the field is not affected either in direction or magnitude while the velocity component normal to the field continually changes direction, remaining constant in magnitude. Strict application of this principle leads one to the conclusion that the trajectory of the electron beam 14 in the tube 13 would be helical. However, consideration of the size of tube 13 and the relative size of the normal and parallel components of the initial electron velocity in the beam 14 shows that a helical path will not result in this practical case. The normal component of the velocity, remaining constant in magnitude, but continually changing direction, causes the beam to describe an approximately circular path. The same considerations indicate that the beam 14, when deflected upwardly or downwardly in tube 13, will follow an essentially circular path which, for any given angle of deflection, occurs entirely in a plane.

The curvature of the electron beam 14 which is aimed at the cent-er of screen 20 may be determined empirically by measuring the distance of the point of inpingement of beam 14 from the center of the viewing screen 20 when no deflection waveform is applied to the deflection yoke 15. It is not necessary to actually determine the geometric center of the viewing screen 24) to measure the desired distance. If the point of impingement of the electron beam 14 be noted and the tube be rotated 180, a second point of impingement of the beam 14 on the viewing screen 20 will be observed. One half the distance between the two points of impingement corresponds to the offset of the electron beam 14 due to its curvature. In a practical tube having a beam length of approximately 21", actual measurements have been made to show that the offset of the beam is approximately /2". Of course, to obtain a working value, a number of different tubes such as tube 13 should be tested to reduce experimental errors due to misalignment of the individual tubes.

The deflection center of the electron beam, marked A, the center of given aperture 16 on the shadow mask 17 and the center on the corresponding phosphor dot 18 lie on a curved line. This line represents the trajectory of the electron beam 14,. Curvature of the electron beam 14 is emphasized by the straight dotted line passing from the center of the phosphor dot 18. through the aperture 16 in the shadow mask and intersecting the deflection plane of the deflection yoke 15 at the point B. Point B represents the position of the dot-forming light source which would be used to form the dot 18 and the other similar color generating dots on the viewing screen 20 in the photographic process. described above. The determination of the distance between point A and point B will be explained in detail in connection with FIG. 2; The utilization of point B in the fabrication of a practical viewing screen will be explained in connection with FIG. 4.

Referring to FIG. 2, a practical method of calculation of the offset of the point B may be seen. In FIG. 2, the line GAX represents an ideal electron beam emanating from an electron gun at the point G, passing through the point A in the deflection plane of the tube and impinging on a viewing screen at the point X; the arc GY represents the path of an actual electron beam emanating from an electron gun at the point G, passing through a uniform transverse field and impinging on the viewing screen at the point Y; the line GY represents a chord drawn from the point G to Y; the line BY represents a tangent to the trajectory of an actual electron beam at the point Y intersecting the deflection plane of the tube at the point B; the lines GO and Y0. represent radii of curvature of an actual electron beam; the angle 15 represents the observed displacement of the point of impact of an actual electron beam from the ideal; the line PO represents a construction line drawn from the intersection of the line GAX and the line BY to the intersection of the lines GO and Y0; and the line AB represents the required offset.

To derive a general formula:

(1) Since the triangle OGY is isosceles, angle OGY equals the complement of one-half the angle (2) By construction, the angle XGY is equal to' the complement of the angle OGY.

(3) In the tetragon OGPY, the angle GPY is equal to the supplement of the angle (4) Therefore, the angle XPY is equal to the angle (6) Therefore, PX=GY(tan /2/tan (7) Simplifying, PX=GY(cos /1+cos (9) Since triangle EPA is similar to triangle YPX,

X Y=.5 inch AX: 16 inches GY=21 inches Substituting in Equation 12, AB=.26 inch It should be noted that Equation 12 above, may be simplified still further in a practical case. Since the angle qb is very small, the cosine of the angle 5 is very nearly equal to unity and Equation 12 reduces to the form:

13 AB=XY((2AX-GY)/GY) FIG. 3 shows how the neck shadow is overcome in a color television picture tube embodying the present invention. It should be noted that the electron beam 14a represents the trajectory of an electron beam which has been properly changed by apurity coil to obtain color registration on the viewing screen. The electron beam 14 represents the trajectory of a properly registered electron beam in a, tube constructed according to this invention. It is evident that the electron beam 14a is more likely to be intercepted by the inner wall of the tube in the area D than the electron beam 14. In fact, the point C is ordinarily so close to the area D that the beam 14a is deflected more than the critical angle so that it is partially blocked from reaching the viewing screen 20. The electron beam 14 cannot, be partially blocked since the critical angle is obviously greater than the maximum deflection angle of beam 14.

The raster in a tube embodying this invention is more centrally located. It is evident that the angle which beam 14 makes with the centerline of the tube is greater than the angle the beam 14a makes. This means that for a given angle of deflection the electron beam 14 impinges on the screen at a point closer to the axis than the electron beam 14a. Thus, the raster is shifted farther in the case of electron beams passing through the point C in the manner of beam 14a. It is obvious that a greater correction is required to center such a raster.

The practical difliculties attendant on producing an absolutely uniform magnetic field by a deflection yoke are well-known. Uniform fields are most easily attained over a small area near the geometric center of any coil. In FIG. 3 it may be seen that the electron beam 14 passes through the deflection coil closer to its geometric center than the electron beam 14a. Since the electron beam 14 passes through an inherently more uniform portion of the magnetic field produced by the deflection yoke, the deflection angle of the electron beam 14 may be more closely controlled.

If it be assumed that the electron beam 14 rotates at a constant angular speed about the point A to trace out each horizontal line in the raster in the preferred embodiment of the invention and that the electron beam 14a traces out each horizontal line in the raster of a presently known tube by rotating at a constant angular speed about the point C, it may be seen that the point of impingement of each electron beam on the viewing screen moves at a varying speed as the horizontal line is generated. However, the variations in speed of the point of impingement of the electron beam 14 are less than the variations of the speed of the point of impingement of the beam 14a.

Having determined the amount of offset required, the apparatus shown in FIG. 4 may be conveniently used to form the dots in a manner similar to that disclosed in the cited Perry and Rowe application. Care must be taken when offsetting the point source of light that it be moved in the correct direction. A simple rule to follow is that the light source should always be moved directly opposite to the direction in which the electron beam bends in the finished tube. If a tube is being compensated for use in the northern hemisphere, the position of the light source will always be to the left of the beam deflection center A, looking toward the shadow mask 17 as shown in FIG. 1. The light must be placed in the plane of the deflection center of the electron beam in order that the length of the electron beam will be substantially constant as the raster is formed.

Referring now to FIG. 4, a simplified form of a lighthouse 21 embodying this invention may be seen. The lighthouse contains a small source of light 22 fixed on a support 29 and means for supporting the shadow mask 17 in a unique position at a predetermined distance from the light source 22. The supporting means may conveniently be pins 23 projecting from a fiat top 24 on the lighthouse 21. The viewing screen may be positioned with respect to the shadow mask 17 in exactly the same manner as in the completed tube, using springs 25 and positioners 26. The type of light source 22 is not material so long as the spot size is small as compared with the distance from the light 22 to the shadow mask 17. A carbon arc may be arranged to operate essentially as a point source to give sharp edges on the dots 18.

It has been the practice in the prior art to place the light source 22 at the beam deflection center (A in FIG. 1) of an electron beam in the finished tube. The rays from the light source 22 were assumed to follow roughly the same path as the electron beam in the finished tube. However, such an assumption is erroneous in any practical case for the reasons given hereinbefore. A simple change in the position of the light source 22 may be made to compensate for the curvature of the electron beam path in a practical tube. If the light source 22 is moved laterally a predetermined distance, calculated as explained hereinabove, from the beam deflection center (A in FIG. 1), the position of each phosphor element 18 on the viewing screen 20 will be shifted to correspond exactly with the point of impact of the electron beam (14 in FIG. 1) on the viewing screen 20. The direction in which the light source 22 must be moved has been explained in connection with FIG. 3.

The light source 22 may be permanently located as required for tubes of a given size. The shadow mask 17 is placed in position. The viewing screen 20, after being coated with a phosphor material and photosensitized, is

then placed in position. Alternatively, the shadow mask 17 and the viewing screen 20 may be uniquely positioned with respect to one another to form the same unitary structure they will form in the finished tube and then uniquely positioned on the lighthouse. The dots 18 are formed on the viewing screen 20 by the light from the source 22 shining through the apertures 16 in the shadow mask 17. After exposure, the dots 18 may be developed and the process repeated to form a second and third array of dots such as dot 18 to produce three primary colors in the finished tube. The position of the light source 22 with respect to the shadow mask 17 is changed each time an array of dots 18 is formed. Either the light source 22 or the shadow mask 17 and viewing screen 20 may be moved to accomplish this end. The preferred embodiment shows a set of auxiliary holes 27 into which the pins 23 may be moved, thereby changing the relative positions of the light source 22 and the shadow mask 17 and the viewing screen 20. It should be noted that a third set of auxiliary holes, such as the holes 27, would be required to determine a third position of the shadow mask 17 and the viewing screen 20 with respect to the light 22. Many mechanical variations will immediately suggest themselves as methods of positioning of the light source 22 with respect to the shadow mask 17.

Although the embodiment of the invention shown and described counteracts the eflect of the vertical component of the earths magnetic field, only simple modifications would be required to counteract other magnetic fields, such as the horizontal component of the earths magnetic field. Such fields are of minor importance in so far as color registry is concerned. It should also be noted that the effects of electric fields may also be minimized by adaptations of the techniques disclosed herein. Furthermore, it is obvious to anyone having skill in the art that results similar to those obtained by this invention could be obtained by displacing the neck axis of the tube from the shadow mask and viewing screen. Variations of this invention to compensate for such external fields are believed to be within the scope and spirit of the present invention which should be limited only by the following claims.

What is claimed is:

1. The method of photographically forming the image of a shadow mask on a photosensitized material disposed on the viewing end of a color television picture tube having at least one electron beam producing source therein capable of producing an electron beam within and subject to the earths magnetic field and deflection means for said electron beam operative on an electron beam deflection plane, including the steps of exposing said photosensitized material to light from a light source through the apertures in said shadow mask to form said image, said light source being disposed at a predetermined point in said electron beam deflection plane, said point being displaced a predetermined distance from the intersection of said electron beam with said electron beam deflection plane, in an amount suflicient and opposite to the oflset of said electron beam on said viewing screen to compensate for the effect of the earths magnetic field on the trajectory of said electron beam and removing the unexposed photosensitized material from said viewing end.

2. The method of photographically forming a pattern of luminescent material on the viewing end of a color television picture tube of the aperture mask type which has at least one electron beam apparently originating at a point in an electron deflection plane and moving under the influence of the vertical component of the earths magnetic field in a curved path therefrom through said aperture mask to impinge on said viewing end and, at the same time, causing the position of the individual elements of said pattern to correspond in position with the point of impingement of said electron beam on said viewing end, comprising the steps of applying a luminescent material in a thin layer on said viewing end, saturating said luminescent material with a photosensitive resist material, arranging said viewing end and said aperture mask on a common axis in the same relative position as the two lastnamed parts will be placed in a completed tube, arranging a light source in a plane substantially coincident with said electron deflection plane and spaced from said point a predetermined distance proportional to the displacement on said viewing end of said curved path of said electron beam from a straight line drawn through said point and any aperture of said aperture mask to said viewing end, exposing said saturated material to light from said source through said aperture mask, and developing away unexposed portions of said saturated material.

3. The method of forming three interlaced patterns of luminescent materials on the viewing end of a color television picture tube of the aperture mask type and, at the same time, causing the position of the individual elements of said interlaced patterns to correspond with the points of impingement on said viewing end of three electron beams passing through the apertures in an aperture mask adjacent said viewing end after travelling in separate curved paths from three equally spaced points in a plane spaced from said aperture mask, comprising the steps of, applying a first luminescent material in a thin layer on said viewing end, saturating said first luminescent material with a photosensitive resist material, arranging said viewing end and said aperture mask on a common axis in the same relative position as the two last-named parts will be placed in a completed tube, arranging a light source in a plane substantially coincident with said plane and spaced from the first of said points to cause light from said light source to pass through the apertures of said aperture mask and impinge on said luminescent material at the same angle as the first of said three electron beams in a completed tube, exposing said first luminescent material to light from said first light source, developing away unexposed portions of said first luminescent material, and repeating the foregoing steps for a second and a third time except using a second and a third luminescent material, respectively, and using a second and a third light source, offsetting said second light source from the second of said points in said plane spaced from said aperture mask in a direction parallel to and for a distance equal to the spacing of said first light source from the first of said points and similarly offsetting said third light source from the third of said points.

4. The method of projecting a tri-color luminescent screen for the viewing end of a color television picture tube of the type having an apertured shadow mask and first, second, and third electron beams curved by interaction with the earths magnetic field capable of being simultaneously deflected in a common plane parallel to a plane tangent to the center of said viewing end and displaced therefrom comprising the steps of, applying a coating of a first phosphor to the inner surface of said viewing end, saturating said first phosphor with photosensitive resist material, arranging said viewing end, said shadow mask, and a spot source of light to simulate the positions of said viewing end, said shadow mask, and the intersection of said first electron beam with said common plane in said color television picture tube, offsetting said spot source of light in said common plane to a first predetermined position removed from said intersection of said first electron beam with said common plane in an amount suflicient such that light from said spot source impinges on said saturated first phosphor at the same angle of incidence as said first curved electron beam, exposing portions of said saturated first phosphor material to light through the apertures of said shadow mask from said spot source of light, developing away unexposed portions of said saturated first phosphor material, repeating the above steps using a second phosphor material in conjunction with light from said spot source of light, said spot source of light being offset from the intersection of said second electron beam with said common plane to a second predetermined position in said common plane in an amount suflicient such that light from said spot source of light impinges on said second phosphor material at the same angle of incidence as said second electron beam, and again repeating said steps using a third phosphor material in conjunction with light from said spot source of light, said source being offset from the'intersection of said third electron beam with said common plane to a third predetermined position in said common plane in an amount sufficient to cause light from said spot source to impinge on said third phosphor material at the same angle of incidence as said third curved electron beam.

5. The method of photographically forming a luminescent screen for a color television picture tube of the type having three different light-emitting phosphors on the viewing end thereof, three sources of curved electron beams, and an aperture mask for limiting the impingement of beams from each source to only phosphors of one light-emitting type, which includes the steps of determining the landing points of said curved electron beams on said screen when subject to the influence of the earths magnetic field, photosensitizing said phosphors on the said screen, placing a light source behind said aperture mask at light source locations offset from the locations of the sources of the curved electron beams where light from said light source passing through said aperture mask lands on said landing points of said curved electron beams, exposing said so-photosensitized phosphors to light from said light source, through said aperture mask at the landing points of said curved beams when subject to the influence of the earths magnetic field, and washing away all unexposed photosensitized phosphors.

6. In the manufacture of a color television picture tube having a source of an electron beam at one end and a face plate and apertured shadow mask at the other end, the method of photographically forming a pattern of luminescent areas on the face plate incorporating a correction for the deflecting effect on the electron beam caused by the vertical component of the earths magnetic field comprising the steps of applying a photosensitive, luminescent material on the inner surface of the face plate, arranging said apertured shadow mask with respect to said face plate in substantially the same position as in the finished tube, exposing said photosensitive, luminescent material through said shadow mask with light passing through the respective apertures in said shadow mask along paths tangent to the paths of said electron beam in the finished tube traveling from said source of an electron beam to said face plate under the deflecting influence of the vertical component of the earths magnetic field.

7. In the manufacture of a color television picture tube having a source of an electron beam at one end and a face plate and apertured shadow mask at the other end, the method of photographically forming a pattern of luminescent areas on the face plate incorporating a correction for the deflecting effect on the electron beam caused by the vertical component of the earths magnetic field comprising the steps of applying a photosensitive, luminescent material on the inner surface of the face plate, arranging said apertured shadow mask with respect to said face plate in substantially the same position as in the finished tube, positioning a light source in a plane substantially in the position of the deflection plane of the finished tube, exposing said photosensitive, luminescent material through said shadow mask from light emanating from said light source passing through the respective apertures in said shadow mask along paths tangent to the paths of said electron beam in the finished tube traveling from said source of an electron beam to said face plate under the deflecting influence of the vertical component of the earths magnetic field.

8. The method of compensating for misregister due to the beam-deflecting effect of the vertical component of the earths magnetic field upon the beam electrons in a cathode ray tube of the kind having a horizontal axis and a deflection plane, a mask having a multiplicity of apertures therein through which said beam electrons pass in their transit to preselected elemental areas on the target surface of a nearby screen of the mosaic variety and an electron gun located on the side of said deflection plane opposite from said screen, said method comprising photographically laying-down a mosaic pattern of elemental screen areas upon said target surface with said mask and said screen held in a given relationship to each other and with the aid of light rays projected through said mask from a source located in said deflection plane at a point offset in a horizontal direction from said horizontal axis at a distance AB defined by the formula:

where XY is the distance between the point of impingement of the electrons on the screen when under the influence of the vertical component of the earths magnetic field and the point of impingement of the electrons on the screen in the absence of the vertical component of the earths magnetic field, AX is the distance between the deflection plane and the screen, and

GY is the straight line distance between said electron gun and screen as measured along said horizontal axis,

and thereafter mounting said mask and screen within said tube with said mask and screen positioned with respect to each other in said given relationship.

References Cited by the Examiner OTHER REFERENCES Howe: Introduction to Physics, 1948, 2nd edition, McGraw-Hill Book Co. Inc., N.Y., pp. 536538, copy in Group 170.

Levy et al.: Sylvania Technologist, (July 1953), pp.

NORMAN G. TORCHIN, Primary Examiner.

RALPH G. NILSON, ANDRES H. NIELSEN, PHILIP E. MANGAN, Examiners.

R. W. BAILEY, M. S. GROSS, G. N. WESTBY, A. D.

RICCI, Assistant Examiners. 

1. THE METHOD OF PHOTOGRAPHICALLY FORMING THE IMAGE OF A SHADOW MASK ON A PHOTOSENSITIZED MATERIAL DISPOSED ON THE VIEWING END OF A COLOR TELEVISION PICTURE TUBE HAVING AT LEAST ONE ELECTRON BEAM PRODUCING SOURCE THEREIN CAPABLE OF PRODUCING AN ELECTRON BEAM WITHIN AND SUBJECT TO THE EARTH''S MAGNETIC FIELD AND DEFLECTION MEANS FOR SAID ELECTRON BEAM OPERATIVE ON AN ELECTRON BEAM DEFLECTION PLANE, INCLUDING THE STEPS OF EXPOSING SAID PHOTOSENSITIZED MATERIAL TO LIGHT FROM A LIGHT SOURCE THROUGH THE APERTURES IN SAID SHADOW MASK TO FORM SAID IMAGE, SAID LIGHT SOURCE BEING DISPOSED AT A PREDETERMINED POINT IN SAID ELECTRON BEAM DEFLECTION PLANE, SAID POINT BEING DISPLACED A PREDETERMINED DISTANCE FROM THE INTERSECTION OF SAID ELETRON BEAM WITH SAID ELECTRON BEAM DEFLECTION PLANE, IN AN AMOUNT SUFFICIENT AND OPPOSITE TO THE OFFSET OF SAID ELECTRON BEAM ON SAID VIEWING SCREEN TO COMPENSATE FOR THE EFFECT OF THE EARTH''S MAGNETIC FIELD ON THE TRAJECTORY OF SAID ELECTRON BEAM AND RE,OVING THE UNEXPOSED PHOTOSENSITIZED MATERIAL FROM SAID VIEWING END. 